Following the development of biochemical industries, recently, attention has been increasingly raised toward the biological activities of rare sugars in the field of sugar chemistry (non-patent reference 1), so that demands toward various types of rare sugars conventionally never needed are now occurring. Therefore, the establishment of a method for stably producing these sugars has been desired. The method for producing these sugars may be done by approaches of organic chemistry. Generally, however, the conditions for the production by the approaches are very tough at very low yields of intended sugars, inappropriately as an industrial production method. As a biochemical approach, alternatively, sugar conversion methods with enzymes are listed. As described in the patent reference 1 for industrial production of L-ribose, the present inventors made a success in developing a milestone therefor. The inventors made further investigations regarding the production and utilization of L-ribose isomerase. Based on the partial amino acid sequence of L-ribose isomerase as disclosed in the patent reference 1, the inventors made diverse examinations about the chromosomal DNAs of microorganisms capable of generating the enzyme. Consequently, the inventors found that DNA encoding a series of amino acid sequences including the partial amino acid sequence was successfully cloned from a microorganism of the genus Acinetobacter (patent reference 2). Using the cloned DNA, then, the inventors made attempts to prepare a recombinant polypeptide, so that the inventors confirmed that a polypeptide with the activity of L-ribose isomerase could efficiently be produced.    Patent reference 1: JP-A-10-155480    Patent reference 2: JP-A-2002-253254    Non-patent reference 1: Rare Sugar Congress in Kagawa: 2005. Proc 2nd Symp Int Soc Rare Sugars, Takamatsu, Japan.